add support for responsively activating reader mode via

js/config.js

@@ -276,6 +276,10 @@ export default {
 	// Only applies to presentations in reader mode.
 	readerScrollSnap: 'proximity',
 
+	// Responsively activate the reader mode when we reach the specified
+	// width (in pixels)
+	readerActivationWidth: null,
+
 	// The maximum number of pages a single slide can expand onto when printing
 	// to PDF, unlimited by default
 	pdfMaxPagesPerSlide: Number.POSITIVE_INFINITY,

js/controllers/reader.js

@@ -14,6 +14,8 @@ export default class Reader {
 		this.active = false;
 		this.activatedCallbacks = [];
 
+		this.onScroll = this.onScroll.bind( this );
+
 	}
 
 	/**
@@ -32,7 +34,7 @@ export default class Reader {
 		const slides = queryAll( this.Reveal.getRevealElement(), SLIDES_SELECTOR );
 
 		viewportElement.classList.add( 'loading-scroll-mode', 'reveal-reader' );
-		viewportElement.addEventListener( 'scroll', this.onScroll.bind( this ) );
+		viewportElement.addEventListener( 'scroll', this.onScroll );
 
 		let presentationBackground;
 		if( viewportElement ) {
@@ -113,7 +115,11 @@ export default class Reader {
 
 		this.active = false;
 
-		this.Reveal.getViewportElement().classList.remove( 'reveal-reader' );
+		const viewportElement = this.Reveal.getViewportElement();
+
+		viewportElement.removeEventListener( 'scroll', this.onScroll );
+		viewportElement.classList.remove( 'reveal-reader' );
+
 		this.Reveal.getSlidesElement().innerHTML = this.slideHTMLBeforeActivation;
 		this.Reveal.sync();
 

js/reveal.js

@@ -999,6 +999,18 @@ export default function( revealElement, options ) {
 						}
 					});
 				}
+
+				// Responsively turn on the reader mode if there is an activation
+				// width configured. Ignore if we're configured to always be in
+				// reader mode.
+				if( typeof config.readerActivationWidth === 'number' && config.view !== 'reader' ) {
+					if( size.presentationWidth < config.readerActivationWidth ) {
+						if( !reader.isActive() ) reader.activate();
+					}
+					else {
+						if( reader.isActive() ) reader.deactivate();
+					}
+				}
 			}
 
 			dom.viewport.style.setProperty( '--slide-scale', scale );